1. Field of the Invention
The present invention generally relates to a micro-mechanical structure and a method for manufacturing the same, and more particularly, to a micro-mechanical structure, of which at least a part is configured of a hydrophilic surface in contact with a sacrificial layer to be removed, in order to prevent the micro-mechanical structure from being stuck in the step of removing the sacrificial layer to release the micro-mechanical structure.
2. Discussion of Related Art
Conventionally, micro-mechanical elements are produced by forming a micro-mechanical structure through a surface micro-machining process, that is, through repetitive vapor-deposition and selective etching processes of a structural layer and a sacrificial layer, and then removing only the sacrificial layer to form an air-gap, thus releasing the micro-mechanical structure.
The micro-mechanical structure is vulnerable to an interfacial force and the resulting adhesion, because of a relatively wide surface area in comparison with its volume, and a relatively narrow gap from the neighboring surface. Thus, there occurs a problematic adhesion phenomenon in the process of removing the sacrificial layer to release the micro-mechanical structure. For this reason, preventing such an adhesion phenomenon is very important for improvement of characteristics and yield of the element [References: Tas et al., “Stiction in surface micro-machining,” J. Micromech. Microeng., vol. 6, pp. 385-397, 1996; and Maboudian et al., “Critical Review: Adhesion in surface micro-mechanical structures,” J. Vac. Sci. Technol. B, vol, 15, no. 1, pp. 1-20, January/February. 1997]. Especially, water produced in the process of etching the sacrificial layer to release the micro-mechanical structure is known to cause the adhesion phenomenon of the micro-mechanical structure. This problem will be described below in detail, taking the most general case of using silicon oxide for the sacrificial layer and etching the sacrificial layer with hydrogen fluoride (HF) by way of an example.
In the case of etching the silicon oxide sacrificial layer through a chemical etching process employing HF, the process can be subdivided into a liquid-phase etching process and a vapor-phase etching process according to the state of HF. The vapor-phase etching process, which is developed posterior to the liquid-phase etching process, has much larger industrial utility because of the advantages of: 1) less occurrence of the adhesion phenomenon; 2) high productivity caused by omission of de-ionized water rinsing and drying processes followed in the liquid-phase etching process; and 3) low cost due to use of small amount of a high purity of HF which is expensive and causes environmental pollution.
In the vapor-phase etching process employing HF, a mixture of a HF gas as a reacting gas, and a water vapor or alcoholic gas serving as a catalyst for chemical reaction is generally used [References: U.S. Pat. No. 6,238,580 B1, filed on December 1999, Cole et al.; U.S. Patent Publication No. 2002/0058422 A1, filed on December 2000, Jang et al.]. Methyl alcohol having an evaporation point (64.5° C.), lower than that (100° C.) of water vapor is more used than the water vapor, because the former is effective to prevent the adhesion phenomenon.
On the other hand, the reaction of the silicon oxide sacrificial layer and the HF gas results in silicon fluoride (SiF4) and water (H2O), as in Formula 1. In this case, silicon fluoride having a low evaporation point (−94.9° C.), is discharged in a gas state, while, in the case of water having a high evaporation point (100° C.), some x is discharged in a vapor state, and the remnant 2−x is condensed and left behind in a liquid state [Reference: Helms et al., “Mechanisms of the HF/H2O vapor phase etching of SiO2.”].SiO2(s)+4HF(g)→xH2O(g)+(2−x)H2O(L)  Formula 1.
FIGS. 1 to 3 are conceptual views for explaining a micro-mechanical structure where an adhesion phenomenon occurs in the conventional process of removing the silicon oxide sacrificial layer through the HF vapor-phase etching process.
A sample is prepared by forming, on a substrate 11, a silicon oxide sacrificial layer 22 and micro-mechanical structures 31a and 31b taking a cantilever shape. A HF vapor-phase etching process is performed to the sample, so that an air-gap g is formed by removal of the silicon oxide sacrificial layer 22. At this point, water in a liquid state is formed in a shape of islands 24 at a contact angle of θc<90° on surfaces of the substrate 11 and the micro-mechanical structures 31a and 31b, both of which are formed of a hydrophilic material. Some of the water islands 24 get in contact with each other, thereby building up a water bridge 25 connecting the substrate 11 and the micro-mechanical structures 31a and 31b. In this situation, the water bridge causes a capillary force F, as expressed in Equation 1, to be exerted between the substrate and the micro-mechanical structure.
                    F        =                              2            ⁢            A            ⁢                                                  ⁢                          γ              la                        ⁢            cos            ⁢                                                  ⁢                          θ              c                                g                                    Equation        ⁢                                  ⁢        1            
wherein, g is the height of the water bridge, namely, the thickness of air-gap, γ1a is the surface tension of water in the air, and θc is the contact angle of water on a solid surface.
In this case, the capillary force has a positive value, that is, serves as an attractive force, because θc is less than 90°. If the capillary attraction becomes larger than the co-efficient of elasticity which is required to deform the micro-mechanical structure, the micro-mechanical structures 31a and 31b are bent to the substrate, thereby sticking temporarily to it. Then, even when all the liquefied water is evaporated, no air-gap g remains between the substrate 11 and the micro mechanical structures 31a and 31b. Accordingly, both of them are permanently stuck by a van der Waals force acting between them.
As described above, the liquefied water, which remains on the substrate and the micro-mechanical structure, causes a problem that the micro-mechanical structure sticks to a base structure such as the substrate [References: Offenberg et al., “Vapor HF etching for sacrificial oxide removal in surface micromachining, Electrochemical Soc. Fall Meet., vol. 94, no. 2, pp. 1056-1057, October 1994; Lee et al., “Dry release for surface micromachining with HF vapor-phase etching,” J. MEMS, vol. 6, no 3, September 1997]. To prevent the liquefied water from being generated, the temperature of the substrate should be increased, while the pressure of reaction should be decreased. However, this remarkably reduces an etching speed of silicon oxide, which results in great reduction in productivity.